RF Filter

ABSTRACT

An RF filter is disclosed. In an embodiment, the RF filter includes series-interconnected basic elements, each basic element having an electroacoustic resonator and impedance converters interconnected in series between the basic elements, wherein the impedance converters are impedance inverters and/or admittance inverters, and wherein the resonators of the basic elements are either only series resonators or only parallel resonators.

This patent application is a national phase filing under section 371 ofPCT/EP2015/068485, filed Aug. 11, 2015, which claims the priority ofGerman patent application 10 2014 111 912.6, filed Aug. 20, 2014, eachof which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to RF filters which can be used, e.g., in portablecommunication devices.

BACKGROUND

Portable communication devices, e.g., mobile radio devices, in themeantime can enable communication in a multiplicity of differentfrequency bands and for a multiplicity of different transmissionsystems. To that end, they generally comprise a multiplicity of RFfilters each provided for the corresponding frequency and thecorresponding transmission system. Although modern RF filters in themeantime can be produced with small dimensions, on account of theirmultiplicity and the complexity of their interconnection the front-endmodules in which the filters are arranged are nevertheless relativelylarge and their production is complex and expensive.

Tunable RF filters could remedy this. Such filters have a centerfrequency that is adjustable, for which reason a tunable filter, inprinciple, can replace two or more conventional filters. Tunable RFfilters are known, e.g., from the documents US 2012/0313731 A1 or EP2530838 A1. In this case, the electroacoustic properties of resonatorsoperating with acoustic waves are altered by tunable impedance elements.

The paper “Reconfigurable Multiband SAW Filters for LTE Applications”,IEEE SiRF 2013, pp. 153-155, by Lu et al., discloses filters that arereconfigurable by means of switches.

What is problematic about known tunable RF filters, however, is, inparticular, the fact that the tuning itself alters important propertiesof the filters. In this regard, e.g., the insertion loss, the inputimpedance and/or the output impedance change(s) during tuning.

SUMMARY OF THE INVENTION

Embodiments of the invention provide RF filters which enable tuningwithout altering other important parameters and which make available tothe person skilled in the art additional degrees of freedom whendesigning filter modules.

In various embodiments the RF filter comprises series-interconnectedbasic elements each having an electroacoustic resonator. The filterfurthermore comprises impedance converters interconnected in seriesbetween the basic elements. The impedance converters are impedanceinverters and/or admittance inverters. The resonators of the basicelements are either only series resonators or only parallel resonators.

Basic elements in RF filters are known from ladder-type structures, forexample, where a basic element comprises a series resonator and aparallel resonator. A plurality of such basic elements connected inseries substantially brings about the filter effect if the resonantfrequencies and the antiresonant frequencies of the series and parallelresonators are tuned suitably in relation to one another.

The basic elements present here can be interpreted in this regard to acertain extent as halved basic elements of a ladder-type circuit.

Impedance inverters or admittance inverters are appropriate as impedanceconverters. While an impedance converter transforms an arbitrarytransformation of a load impedance into an input impedance, the effectof the impedance inverter or admittance inverter is distinctlyconcretized. Impedance inverters or admittance inverters can bedescribed as follows using the auxiliary aids for two-port networks.

The chain matrix having the matrix elements A, B, C, D describes theeffect of a two-port network connected to a load by its output port, bysaid chain matrix stipulating how a voltage U_(L) dropped across a loadand a current I_(L), flowing through a load are transformed into avoltage U_(IN) present at the input port and a current I_(IN) flowinginto the input port:

$\begin{matrix}{\begin{pmatrix}U_{IN} \\I_{IN}\end{pmatrix} = {\begin{pmatrix}A & B \\C & D\end{pmatrix}\begin{pmatrix}U_{L} \\I_{L}\end{pmatrix}}} & (1)\end{matrix}$

The impedance Z is defined here as the ratio between voltage andcurrent:

$\begin{matrix}{Z = \frac{u}{i}} & (2)\end{matrix}$

A load impedance Z_(L) is thus transformed into an input impedanceZ_(IN):

$\begin{matrix}{Z_{IN} = \frac{{AZ}_{L} + B}{{CZ}_{L} + D}} & (3)\end{matrix}$

From outside, therefore the load impedance Z_(L) looks like the inputimpedance Z_(IN).

An impedance inverter is then characterized by the following chainmatrix:

$\begin{matrix}{\begin{pmatrix}A & B \\C & D\end{pmatrix}_{K} = \begin{pmatrix}0 & {iK} \\{i/K} & 0\end{pmatrix}} & (4)\end{matrix}$

It follows from this that

$\begin{matrix}{Z_{IN} = \frac{K^{2}}{Z_{L}}} & (5)\end{matrix}$

The impedance is inverted. The proportionality factor is K².

An admittance inverter is characterized by the following chain matrix:

$\begin{matrix}{\begin{pmatrix}A & B \\C & D\end{pmatrix}_{J} = \begin{pmatrix}0 & {i/J} \\{iJ} & 0\end{pmatrix}} & (6)\end{matrix}$

It follows from this for the admittance Y that:

$\begin{matrix}{Y_{IN} = \frac{J^{2}}{Y_{L}}} & (7)\end{matrix}$

The admittance is inverted. The proportionality factor is J².

It has been found that joint presence of parallel resonators and seriesresonators has distinct effects on the variability of importantparameters when tuning the RF filter. It has furthermore been found thattuning has less influence on said parameters if only one type ofresonators is present. Therefore, if only series resonators or onlyparallel resonators are present, the RF filter behaves more stably withregard to the insertion loss, the input impedance and/or the outputimpedance during tuning. It has additionally been found that theabovementioned impedance converters are suitable for causing seriesresonators to appear as parallel resonators, and vice versa. Inparticular, a series interconnection of two impedance inverters with aseries resonator therebetween looks like a parallel resonator to thecircuit environment thereof. A series interconnection of two admittanceinverters with a parallel resonator therebetween looks like a seriesresonator to the circuit environment thereof.

These series interconnections thus make it possible to create RF filtercircuits which are better tunable.

It is thus possible to configure the RF filter such that the impedanceconverters are impedance inverters and the resonators are seriesresonators.

Such filters do not require any parallel resonators. If the filters areconfigured as bandpass filters or as band-stop filters, then thesegenerally have a steep right-hand edge. The filter can be used in aduplexer. Preferably as a transmission filter owing to the steepright-hand edge. Specifically if the transmission band is below thereception band. If the relative arrangement of transmission band andreception band is interchanged, the filter with series resonators ispreferably in the reception filter.

It is furthermore also possible to configure the RF filter such that theimpedance converters are admittance inverters and the resonators areparallel resonators.

Such filters do not require any series resonators. If the filters areconfigured as bandpass filters or as band-stop filters, then thesegenerally have a steep left-hand edge. The filter can also be used in aduplexer. Preferably as a reception filter owing to the steep left-handedge. Specifically if the reception band is above the transmission band.If the relative arrangement of transmission band and reception band isinterchanged, the filter with series resonators is preferably in thetransmission filter.

It is possible for the impedance converters to comprise both capacitiveelements and inductive elements as impedance elements. However, it isalso possible for the impedance converters to comprise only capacitiveelements or only inductive elements. The impedance converters thenconsist only of passive circuit elements. Particularly if the impedanceconverters comprise only few or no inductive elements at all, they caneasily be realized as structured metallizations in metal layers of amultilayer substrate.

It is possible for the impedance converters to comprise phase shifterlines in addition to inductive or capacitive elements. However, it isalso possible for the impedance converters to consist of phase shifterlines. Phase shifter lines, too, can be integrated simply and with acompact construction in a multilayer substrate.

It is possible for the filter to be described by a symmetricaldescription matrix B.

There are filter circuits which are fully described by a descriptionmatrix B. The matrix B contains matrix elements which characterize theindividual circuit components of the filter.

A filter circuit which comprises three series-interconnected resonatorsR₁, R₂, R₃ and is interconnected with a source impedance ZS on the inputside and with a load impedance ZL on the output side would have thefollowing form:

$\begin{matrix}{B = \begin{pmatrix}Z_{S} & 0 & 0 & 0 & 0 \\0 & R_{1} & 0 & 0 & 0 \\0 & 0 & R_{2} & 0 & 0 \\0 & 0 & 0 & R_{3} & 0 \\0 & 0 & 0 & 0 & Z_{L}\end{pmatrix}} & (8)\end{matrix}$

The circuit would not operate, however, as a bandpass filter.

If the two outer series resonators are masked by impedance inverterssuch that they appear in each case as parallel resonators, then astructure is obtained which behaves like a ladder-type structure andwhich is described by the following description matrix.

$\begin{matrix}{B = \begin{pmatrix}Z_{S} & K_{S\; 1} & 0 & 0 & 0 \\K_{S\; 1} & R_{1} & K_{12} & 0 & 0 \\0 & K_{13} & R_{2} & K_{22} & 0 \\0 & 0 & K_{23} & R_{3} & K_{3\; L} \\0 & 0 & 0 & K_{2\; L} & Z_{L}\end{pmatrix}} & (9)\end{matrix}$

Here K_(S1) denotes the impedance inverter between the source impedanceZ_(S) and the first resonator. K₁₂ denotes the impedance inverterbetween the first and second resonators. Generally, the indices of thevariables of the inverters denote the resonators between which thecorresponding inverters are arranged. It holds true that B_(ij)=B_(ji),i.e., the matrix is symmetrical with respect to its diagonals. Thefilter circuit associated with equation (9) is shown in FIG. 1. Theresonators are described by variables on the diagonal of the matrix. Theimpedance converters are described by variables on the secondarydiagonals directly above and below the diagonal.

It is possible for the filter to comprise a second impedance converterconnected in parallel with a segment of the filter. The segmentcomprises a series connection having a basic element and two impedanceconverters.

The description matrix then contains entries above the upper secondarydiagonal and below the lower secondary diagonal.

It is possible for at least one of the resonators of the basic elementsto be tunable.

In principle and particularly if one of the resonators is tunable, BAWresonators (BAW=Bulk Acoustic Wave), SAW resonators (SAW=SurfaceAcoustic Wave), GBAW resonators (GBAW=Guided Bulk Acoustic Wave) and/orLC resonators are appropriate. Resonator elements operating withacoustic waves substantially have an equivalent circuit diagram with aparallel connection formed by a capacitive element C_(O), on the onehand, and a series connection having an inductive element L₁ and acapacitive element C₁, on the other hand. Such a resonator element hasits resonant frequency at

$\begin{matrix}{\omega_{0} = \sqrt{\frac{1}{L_{1}C_{1}}}} & (10)\end{matrix}$

and its antiresonant frequency at

$\begin{matrix}{\omega_{p} = {{\omega_{0}\sqrt{1 + \frac{C_{1}}{C_{0}}}} = {\sqrt{\frac{1}{L_{1}C_{1}}}\sqrt{1 + \frac{C_{1}}{C_{0}}}}}} & (11)\end{matrix}$

If the resonator also comprises, besides the resonator element, tunableelements such as tunable inductive or capacitive elements connected inseries and/or in parallel with the resonator element, then a resonatorhaving a variable frequency behavior is formed. In this case, theresonant frequency is dependent on L₁ and C₁ but not on C_(O). Theantiresonance is additionally dependent on C_(O). By varying theimpedance of the tunable impedance elements, C_(O) and L₁ of theequivalent circuit diagram can be varied independently of one another.The resonant frequency and the antiresonant frequency can thus be setindependently of one another.

As an alternative to resonators having resonator elements whosecharacteristic frequencies are variable by means of tunable impedanceelements, or in addition thereto, a tunable resonator can comprise anarray of resonator elements, each element of which is coupleable to theresonator or disconnectable from the resonator by means of switches. Anarray of m resonator elements per tunable resonator is then involved. Itis thus possible to construct RF filters which—depending on thepresently active resonator element—can realize m different filtertransmission curves. In this case, each of the m resonators can beassigned to exactly one filter transmission curve. However, it is alsopossible for a plurality of simultaneously active resonator elements tobe assigned to a filter transmission curve. In this regard, m resonatorelements enable up to m! (factorial of m) different filter transmissioncurves. In this case, m can be 2, 3, 4, 5, 6, 7, 8, 9, 10 or even more.If the resonator elements are connected in parallel, 2^(m) differentfilter transmission curves are possible.

In this case, the switches can be switches created in a semiconductordesign such as CMOS switches (CMOS=Complementary metal oxidesemiconductor), GaAs (gallium arsenide) based switches or JFET switches(JFET=Junction FET [FET=Field Effect Transistor]). MEMS switches(MEMS=Microelectromechanical System) are also possible and makeexcellent linear properties available.

It is therefore possible for all the resonators to be tunable todifferent frequency bands.

It is possible, in particular, for the tunability of the resonators toenable a compensation of a temperature fluctuation, an adjustment of thefilter with regard to an impedance matching, an adjustment of the filterwith regard to an insertion loss or an adjustment of the filter withregard to an isolation.

It is furthermore possible for each resonator to comprise the samenumber of resonator elements which are controllable via switchesaddressable via an MIPI interface (MIPI=Mobile Industry ProcessorInterface).

It is possible for one or more impedance converters to comprise orconsist of passive impedance elements. The impedance converter cantherefore comprise two parallel capacitive elements and one parallelinductive element. This is taken to mean transverse branches, e.g.,relative to ground, which contain a corresponding capacitive andinductive element, respectively.

It is also possible for an impedance converter to comprise threeparallel capacitive elements.

It is also possible for an impedance converter to comprise threeparallel inductive elements.

It is also possible for an impedance converter to comprise two parallelinductive elements and one parallel capacitive element.

Computationally it may arise that individual impedance elements have tohave negative impedance values, e.g., negative inductances or negativecapacitances. However, negative impedance values are unproblematic atleast if the corresponding impedance elements are to be interconnectedwith other impedance elements of the RF filter, such that theinterconnection with the other elements has positive impedance valuesagain in total. In this case, the interconnection of the elementsactually provided would be replaced by the element having a positiveimpedance value.

It is furthermore possible for the RF filter to comprise twoseries-interconnected basic elements and a capacitive elementinterconnected in parallel with the two series-interconnected basicelements.

It is possible for the RF filter to comprise a signal path, fourcapacitive elements in the signal path, six switchable resonators eachhaving a resonator element and a switch interconnected in seriestherewith in a transverse branch relative to ground, and an inductiveelement connected in parallel with two of the four capacitive elements.

Hereinafter, important principles are explained and a non-exhaustiveenumeration of exemplary and schematic circuits illustrates centralaspects of the RF filter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1 shows an RF filter F having three resonators and four impedanceconverters,

FIG. 2 shows a filter having three resonators and two impedanceconverters,

FIG. 3 shows a duplexer D having a transmission filter TX and areception filter RX, which are interconnected with an antenna via animpedance matching circuit,

FIG. 4 shows an RF filter F exhibiting interconnection of centrally aseries resonator S and peripherally in each case a series resonator withtwo impedance converters,

FIG. 5 shows an RF filter F which comprises exclusively parallelresonators as resonators used,

FIG. 6 shows an RF filter F in which an impedance converterinterconnects a first resonator directly with a third resonator,

FIG. 7 shows an RF filter F in which an admittance inverter directlyinterconnects a first resonator with a third resonator,

FIG. 8 shows an RF filter having tunable resonators,

FIG. 9A to FIG. 9K show various embodiments of tunable resonators,

FIG. 10A shows a tunable resonator having series resonator elements thatare activatable by switches,

FIG. 10B shows a tunable resonator having parallel resonators that areactivatable by means of switches,

FIG. 11A to FIG. 11F show various embodiments of an impedance inverter,

FIG. 12A to FIG. 12F show various embodiments of an admittance inverter,

FIG. 13A to FIG. 13C show various abstraction stages in the design of anRF filter,

FIG. 14A to FIG. 14H show various concrete embodiments of an RF filterhaving two tunable series resonators and three impedance converters,

FIG. 15A to FIG. 15H show configurations of an RF filter having twotunable resonators, three impedance converters and in each case abridging capacitive element,

FIG. 16 shows the insertion loss of a resonator (A) and of acorresponding bandpass filter (B),

FIG. 17 shows the transmission curves of the RF filter from FIG. 16,wherein tunable impedance elements are altered in terms of theirimpedance in order to obtain a new position of the passband B,

FIG. 18 shows the admittance (A) of a resonator and the insertion loss(B) of a corresponding bandpass filter with admittance inverters,

FIG. 19 shows the RF filter regarding FIG. 18, wherein impedance valuesof tunable impedance elements were varied in order to obtain an alteredposition of the passband,

FIG. 20 shows insertion losses (B, B′) of an RF filter in whichdifferent frequency positions of the passband are obtained by the tuningof resonators,

FIG. 21 shows different transmission curves (B, B′) of an RF filterhaving parallel resonators and admittance inverters in which differentimpedance values bring about different positions of the passband,

FIG. 22 shows insertion losses of a tunable duplexer: the curves B₁ andB₃ here denote a tunable transmission frequency band. The curves B₂ andB₄ represent the insertion losses of an adjustable reception frequencyband,

FIG. 23 shows a possible filter circuit,

FIG. 24 shows one possible form of the integration of circuit componentsin a device,

FIG. 25 shows transfer functions of a tunable filter according to FIG.23.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an RF filter circuit F having three resonators and fourimpedance converters IW. The central resonator here represents a basicelement GG. The central resonator can be a parallel resonator P or aseries resonator S. The two impedance converters IW surrounding thefirst resonator have the effect that the resonator looks like a seriesresonator or like a parallel resonator toward the outside. If thecentral resonator is a parallel resonator, then the first resonator canalso be a parallel resonator that looks like a series resonator towardthe outside. Correspondingly, the third resonator would then also be aparallel resonator that looks like a series resonator toward theoutside. Conversely, the central resonator can be a series resonator S.The two outer resonators would then also be series resonators that looklike parallel resonators toward the outside. In this regard, using theimpedance converters IW a filter structure similar to a ladder-type canbe obtained, even though exclusively series resonators or even thoughexclusively parallel resonators are used.

FIG. 2 shows a filter circuit in which the central resonator is maskedby the impedance converters IW surrounding it such that the filter lookslike an alternating sequence of parallel and series resonators towardthe outside, even though only one type of resonator is used.

FIG. 3 shows a duplexer D in which both the transmission filter TX andthe reception filter RX comprise series interconnections of impedanceconverters and resonators which are interconnected with one another suchthat only one type of resonators is necessary per filter. Since seriesresonators are suitable for forming a steep right-hand filter edge of apassband and since transmission frequency bands are generally below thereception frequency bands in terms of frequency, it is advantageous touse series resonators in the transmission filter TX. Parallel resonatorscould analogously be used in the reception filter RX. If thetransmission frequency band is above the reception frequency band, thenseries resonators in the reception filter and parallel resonators in thetransmission filter would correspondingly be advantageous.

The filters TX, RX are interconnected with an antenna ANT via animpedance matching circuit IAS. From the point of view of the impedancematching circuit IAS, each of the two filters TX, RX looks like aconventional ladder-type filter circuit, such that in practice noadditional outlay is necessary when designing the other circuitcomponents such as antenna and impedance matching circuit.

FIG. 4 correspondingly shows an embodiment in which the centralresonator is embodied as a series resonator S. By virtue of the effectof the impedance converters IW, a series resonator element can in eachcase be used in the two outer resonators as well, even though thecombination of impedance converters and series resonator looks and ismanifested like a parallel resonator P toward the outside. In order tocause series resonators to look like parallel resonators toward theoutside, impedance inverters K are preferably used.

In contrast thereto, FIG. 5 shows an embodiment of an RF filter F inwhich exclusively parallel resonators are used. Using admittanceinverters J as embodiments of the impedance converters IW, the two outerparallel resonators appear as series resonators S. Together with thecentral, middle resonator, a parallel resonator P, the RF filter F formsa quasi-ladder-type structure.

FIG. 6 shows an embodiment in which the two outer resonators aredirectly interconnected via a further impedance converter, e.g., animpedance inverter. The direct interconnection of the outer resonatorsvia a further impedance converter represents a new degree of freedom viawhich an RF filter can be further optimized.

FIG. 7 shows, for example, an embodiment of an RF filter F which usesparallel resonators and admittance inverters J. In this case, the twoouter resonators are also directly interconnected with one another via afurther admittance inverter J.

FIG. 8 shows a possible embodiment of an RF filter in which theresonators are tunable.

FIG. 9 shows a possible embodiment of a tunable resonator R. Theresonator R comprises a resonator element RE. The resonator element REhere may be a resonator element operating with acoustic waves. Acapacitive element CE is interconnected in parallel with the resonatorelement RE. A further capacitive element CE is interconnected in serieswith the parallel interconnection. The two capacitive elements CE aretunable, that is to say that their capacitance can be adjusted.Depending on capacitive elements used, the capacitance can be adjustedcontinuously or in discrete values. If the capacitive elements comprisevaractors, for example, then the capacitance can be continuouslyadjusted by applying a bias voltage. If a capacitive element CEcomprises a bank of capacitive individual elements which can be drivenindividually by means of one or more switches, then the capacitance ofthe corresponding capacitive element CE can be adjusted in discretesteps.

FIG. 9B shows an alternative possibility of a resonator R in which theseries interconnection of a tunable capacitive element CE with aresonator element RE is interconnected in series with a tunableinductive element IE.

FIG. 9C shows a possible embodiment of a tunable resonator R in which aresonator element RE is interconnected in parallel with a tunableinductive element IE.

This parallel connection is interconnected in series with a tunablecapacitive element CE.

FIG. 9D shows a further alternative embodiment for a tunable resonatorR. In this case—in comparison with FIG. 9C—the parallel connection isinterconnected in series with a tunable inductive element IE.

FIG. 9E shows a further alternative embodiment of a tunable resonator inwhich a resonator element RE is only interconnected in parallel with atunable capacitive element CE.

FIG. 9F shows a further alternative embodiment of a tunable resonator R.In this case, a resonator element RE is interconnected in parallel witha tunable inductive element IE.

FIGS. 9E and 9F show relatively simple embodiments of a tunableresonator R. FIGS. 9A to 9D show embodiments of a tunable resonator Rwhich enable further degrees of freedom in tuning by means of a furthertunable element. In this regard, the embodiments shown can beinterconnected in series or in parallel with further capacitive andinductive elements having a fixed impedance or variable impedance, inorder to obtain additional degrees of freedom, e.g., for a wider tuningrange.

FIG. 9G shows an embodiment of a tunable resonator R in which theresonator element RE is interconnected in parallel with a seriesinterconnection comprising an inductive element IE and a tunablecapacitive element CE.

FIG. 9H shows an embodiment of a tunable resonator R in which theresonator element RE is interconnected in parallel with a parallelinterconnection comprising an inductive element IE and a tunablecapacitive element CE.

FIG. 9I shows an embodiment of a tunable resonator R in which theresonator element RE is interconnected in series with a seriesinterconnection comprising an inductive element IE and a tunablecapacitive element CE.

FIG. 9J shows an embodiment of a tunable resonator R in which theresonator element RE is interconnected, on the one hand, in series witha series interconnection comprising an inductive element IE and atunable capacitive element CE and, on the other hand, in parallel with aparallel interconnection comprising an inductive element IE and atunable capacitive element CE.

FIG. 9K shows an embodiment of a tunable resonator R in which theresonator element RE is interconnected, on the one hand, in series witha series interconnection comprising a tunable inductive element IE and atunable capacitive element CE and, on the other hand, in parallel with aparallel interconnection comprising a tunable inductive element IE and atunable capacitive element CE.

It furthermore holds true that switchable tunable elements, e.g.,varactors that can be switchably connected in by means of switches, arealso possible besides continuously tunable elements such as varactorsand switchable elements having a constant impedance.

It holds true even more generally that in a resonator the resonatorelement can be interconnected in series with a series network and inparallel with a parallel network. In this case, the series network andthe parallel network can each comprise impedance elements having a fixedor variable impedance.

FIG. 10 shows an additional possible embodiment of a tunable resonator Rcomprising a multiplicity of resonator elements RE and a multiplicity ofswitches SW. In this case, FIG. 10A shows resonator elements RE whichare interconnected in series in the signal path SP. A tunable seriesresonator is thus illustrated. By individually opening and closing theindividual switches SW, it is possible to couple, in an individuallyadjustable manner, specific resonator elements RE into the signal pathSP. If the tunable resonator R in FIG. 10A comprises m resonatorelements RE, then 2^(m) different switching states can be obtained.

FIG. 10B shows an embodiment of a tunable resonator R in which resonatorelements interconnect the signal path SP with ground. Since the order ofthe individual resonator elements RE in which they are interconnectedwith the signal path SP is relevant, in principle, m! (factorial)different resonator states can be obtained.

FIGS. 11A to 11F indicate various embodiments of an impedance inverter.

FIG. 11A thus shows a form of an impedance converter which represents animpedance inverter. Two capacitive elements are interconnected in seriesin the signal path. A capacitive element interconnects the commoncircuit node of the two capacitive elements in the signal path withground. The capacitive elements in the signal path computationallyacquire a negative capacitance −C. The capacitive element in theparallel path relative to ground computationally acquires a positivecapacitance C.

As already described above, the capacitance values arise merely from thecalculation specifications for two ports. Thus, on no account does theT-circuit shown in FIG. 11A need to be realized in this way in a circuitenvironment. Rather, the capacitive elements having a negativecapacitance in the series path can be combined with further capacitiveelements having a positive capacitance which are additionallyinterconnected in the series path, such that overall in each case one ormore capacitive elements having a positive capacitance are obtained.

The same applies to the embodiments in FIGS. 11B, 11C and 11D and to theembodiments of the admittance inverters in FIGS. 12A, 12B, 12C and 12D.

FIG. 11B shows a T-circuit composed of inductive elements, wherein thetwo inductive elements interconnected in series in the signal pathpurely formally have to have a negative inductance.

FIG. 11C shows a form of an impedance inverter having a Pi-circuit witha capacitive element having a negative capacitance in the series pathand two capacitive elements having a positive capacitance in each casein a parallel path.

FIG. 11D shows an embodiment of an impedance inverter in Pi-form inwhich the inductance of the inductive element in the signal path isnegative. The inductances of the inductive elements in the correspondingtwo parallel paths are positive.

FIG. 11E shows an embodiment of an impedance inverter having a phaseshifter circuit and an inductive element having the inductance L. Inthis case, the phase shifter circuit preferably has the characteristicimpedance of the signal line Z_(O). The phase offset Θ as a result ofthe phase shifter circuit is set in a suitable manner.

In this regard, Θ in the case of an impedance inverter can bedetermined, e.g., by the equation

$\Theta = {{- \tan^{- 1}}{\frac{2\; X}{Z_{0}}.}}$

In this case,

$X = \frac{K}{1 - \left( \frac{k}{Z_{0}} \right)^{2}}$

and K is determined by

$Z_{in} = {\frac{K^{2}}{Z_{l}}.}$

In the case of an admittance inverter, the following can hold true:

$\Theta = {{- \tan^{- 1}}{\frac{2\; B}{Y_{0}}.}}$

In this case,

$B = \frac{J}{1 - \left( \frac{J}{Y_{0}} \right)^{2}}$

and J is determined by

$Y_{in} = {\frac{J^{2}}{Z_{l}}.}$

Analogously to FIG. 11E, FIG. 11F shows an alternative embodiment inwhich the inductive element is replaced by a capacitive element havingthe capacitance C.

FIGS. 12A to 12F show embodiments of an admittance inverter.

FIG. 12A shows an embodiment of an admittance inverter in aT-configuration in which the two capacitive elements in the series pathhave positive capacitances. The capacitive element in the parallel pathnominally has a negative capacitance.

FIG. 12B shows an embodiment of an admittance inverter in aT-configuration, wherein two inductive elements having the inductance Lare interconnected in series in the signal path. An inductive elementhaving the negative inductance −L is interconnected in a parallel pathwhich interconnects two electrodes of the inductive elements withground.

FIG. 12C shows an embodiment of an admittance inverter in aPi-configuration, wherein the two capacitive elements in the twoparallel paths have a negative capacitance. The capacitive element inthe signal path has a positive capacitance.

FIG. 12D shows an embodiment of an admittance inverter in aPi-configuration having three inductive elements. The inductive elementin the series path has a positive inductance. The two inductive elementsin the two parallel paths in each case have a negative inductance.

FIG. 12E shows an embodiment of an admittance inverter in which aninductive element having a positive inductance L is interconnectedbetween two segments of a phase shifter circuit. Each segment of thephase shifter circuit has a characteristic impedance Z_(O) and suitablyshifts the phase.

In a manner corresponding to FIG. 12E, FIG. 12F shows an embodiment ofan admittance inverter which is likewise based on phase shiftercircuits. A capacitive element having a positive capacitance C isinterconnected between two segments of a phase shifter circuit.

FIG. 13 shows the use of tunable resonators R together with impedanceconverters IW. In this case, the resonator can be a series resonator.Through the use of impedance inverters K as impedance converters IW, acombination of two impedance converters IW and a series resonatorinterconnected therebetween produces overall a parallel resonator.

If the impedance converters IW in FIG. 13A are replaced by impedanceinverters such as are known, for example, from FIGS. 11A to 11F, e.g.,11A, then the circuit structure in FIG. 13B is obtained. The capacitiveelements having a negative capacitance appear to be problematic.However, if it is taken into account that the resonators R themselveshave properties of capacitive elements having a positive capacitance,then the need for capacitive elements having a negative capacitancewhich are interconnected directly with the resonator elements isobviated. This is shown in FIG. 13C.

If capacitive elements interconnected in the circuit environment of theRF filter are furthermore taken into account, then the need for theperipheral capacitive elements having a negative capacitance in FIG. 13Cis also obviated. Overall, a circuit structure as shown in FIG. 14A isthen obtained. Even if an external circuit environment of the RF filterdoes not provide a possibility for compensation of the negativecapacitances −C in FIG. 13, the negative capacitance can be compensatedfor by the positive capacitance of the capacitive element in theparallel path.

FIG. 14A thus shows an RF filter circuit which is simple to produce andwhich has two tunable resonators and three impedance elements, theimpedance of which is chosen such that one of the two resonators acts asa parallel resonator. FIG. 14A thus substantially shows a basic elementof a ladder-type filter circuit, even though only series resonators areused.

FIG. 14B shows an alternative to the RF filter in FIG. 14A, since theinductive element L between the resonators is replaced by a capacitiveelement C and the capacitive element in the load-side parallel path isreplaced by an inductive element.

FIG. 14C shows a further embodiment of an RF filter having tworesonators, wherein three inductive elements in each case in a parallelpath are interconnected.

FIG. 14D shows a possible embodiment of an RF filter in which the twoleft impedance elements are formed by inductive elements and the rightimpedance element is formed by a capacitive element.

FIG. 14E shows an embodiment in which the outer two impedance elementsare formed by inductive elements and the central impedance element isformed by a capacitive element.

FIG. 14F shows an embodiment in which the two right impedance elementsare formed by capacitive elements and the left impedance element isformed by an inductive element.

FIG. 14G shows an embodiment in which the two right impedance elementsare formed by inductive elements and the left impedance element isformed by a capacitive element.

FIG. 14H shows an embodiment in which all three impedance elements areformed by capacitive elements.

FIGS. 15A to 15H show further alternatives of the RF filters in FIGS.14A to 14H, wherein a further impedance element directly interconnectsthe signal input and the signal output with one another. A bridginginductive element or other embodiments of impedance converters can beused as an alternative to the bridging capacitive element.

FIG. 16 shows the admittance of a resonator (curve A) and the transferfunction of an RF filter having such a resonator (curve B). Serialcapacitive elements have a value of 2.4 pF. Parallel capacitive elementshave a value of 0.19 pF.

FIG. 17 shows the corresponding curves, wherein serial tunablecapacitances have been set to a capacitance value of 30 pF and paralleltunable capacitances have been set to a capacitance value of 3.7 pF. Theimpedance converters of the filters associated with FIGS. 16 and 17 areimpedance inverters. The resonators are series resonators.

In comparison therewith, FIGS. 18 and 19 show corresponding curves of RFfilters having admittance inverters and parallel resonators. In thiscase, FIG. 18 shows the characteristic curves of a filter in whichserial tunable capacitances have a value of 2.4 pF and parallel tunablecapacitive elements have a value of 0.19 pF.

FIG. 19 shows the corresponding curves of the RF filter in which theserial tunable capacitances have a value of 30 pF and the paralleltunable capacitances have a value of 3.7 pF.

FIG. 20 shows insertion losses of bandpass filters having admittanceinverters and parallel resonators. The filter comprises tunableresonators which are tuned by adjustable capacitances of capacitiveelements once to the reception band 17 and band 5, respectively. In thiscase, the resonators comprise resonator elements that are coupleable bymeans of switches, as shown in FIG. 10B.

FIG. 21 here shows transmission curves of an RF filter having impedanceinverters and series resonators, wherein the tunable values are tunedonce to the transmission frequencies of the band 17 and once to thetransmission frequencies of the band 5. The resonators here compriseresonator elements that are coupleable by means of switches, as shown inFIG. 10A.

FIG. 22 shows the insertion losses of the reception and transmissionfilters of a tunable duplexer, tuned once to band 17 and once to band15.

FIG. 23 shows a possible embodiment of the RF filter. Four capacitiveelements are interconnected in series in the signal path SP. Arespective switchable resonator is interconnected in six transversebranches relative to ground. Each of the switchable resonators comprisesa resonator element and a switch interconnected in series therewith. Aninductive element is connected in parallel with two of the fourcapacitive elements.

FIG. 24 shows how circuit components of the filter circuit canadvantageously be integrated in a multilayered module. The capacitiveelements CE can be realized as MIM capacitors (MIM=metal insulatormetal) together with sections of the signal path in one layer. Theswitches SW can be realized at the bottom of this layer. In a layersituated underneath, plated-through holes can be led which constitutelines of an interface between (semiconductor) switches and the resonatorelements. The resonator elements, e.g., as SAW, BAW, GBAW, . . . etc.elements, can then be arranged below the layer having the interface.

FIG. 25 shows calculated transmission curves for the bands 34 and 39,between which it is possible to switch over by means of switches.

RF filters or duplexers having RF filters can furthermore compriseadditional resonators or impedance elements, in particular tunableimpedance elements.

1-16. (canceled)
 17. An RF filter comprising: series-interconnected basic elements, each basic element having an electroacoustic resonator; and impedance converters interconnected in series between the basic elements, wherein the impedance converters are impedance inverters and/or admittance inverters, and wherein the resonators of the basic elements are either only series resonators or only parallel resonators.
 18. The RF filter according to claim 17, wherein the impedance converters are impedance inverters, and the resonators are series resonators.
 19. The RF filter according to claim 17, wherein the impedance converters are admittance inverters, and the resonators are parallel resonators.
 20. The RF filter according to claim 17, wherein the impedance converters comprise as impedance elements: capacitive elements and inductive elements; only capacitive elements; or only inductive elements.
 21. The RF filter according to claim 17, wherein the impedance converters comprise phase shifter lines.
 22. The RF filter according to claim 17, wherein the RF filter is described by a symmetrical description matrix B where B_(ij)=B_(ji).
 23. The RF filter according to claim 17, further comprising a second impedance converter, wherein the second impedance converter is connected in parallel with a segment of the RF filter, and wherein the segment comprises a series connection having a basic element and two impedance converters.
 24. The RF filter according to claim 17, wherein at least one of the resonators of the basic elements is a tunable resonator.
 25. The RF filter according to claim 24, wherein the tunable resonator comprises a resonator element and a tunable impedance element interconnected in series or in parallel with the resonator element.
 26. The RF filter according to claim 24, wherein the tunable resonator comprises a plurality of resonator elements and a plurality of switches so that each resonator element is individually coupleable to a signal path by a switch.
 27. The RF filter according to claim 17, wherein the impedance converter comprises: two parallel capacitive elements; and one parallel inductive element.
 28. The RF filter according to claim 27, wherein the RF filter comprises two series-interconnected basic elements and a capacitive element interconnected in parallel with the two series-interconnected basic elements.
 29. The RF filter according to claim 17, wherein the impedance converter comprises three parallel capacitive elements.
 30. The RF filter according to claim 29, wherein the RF filter comprises two series-interconnected basic elements and a capacitive element interconnected in parallel with the two series-interconnected basic elements.
 31. The RF filter according to claim 17, wherein the impedance converter comprises three parallel inductive elements.
 32. The RF filter according to claim 31, wherein the RF filter comprises two series-interconnected basic elements and a capacitive element interconnected in parallel with the two series-interconnected basic elements.
 33. The RF filter according to claim 17, wherein the impedance converter comprises: two parallel inductive elements; and one parallel capacitive element.
 34. The RF filter according to claim 33, wherein the RF filter comprises two series-interconnected basic elements and a capacitive element interconnected in parallel with the two series-interconnected basic elements.
 35. The RF filter according to claim 17, wherein the RF filter comprises: a signal path; four capacitive elements in the signal path; six switchable resonators, each switchable resonator having a resonator element and a switch interconnected in series therewith in a transverse branch relative to ground; and an inductive element connected in parallel with two of the four capacitive elements. 